In an effort to increase device densities, many years of research have been devoted to reducing critical dimensions (CDs) in semiconductor devices. Research has resulted in a long felt need to replace traditional gate structures with gates having high-k dielectrics and metal electrodes. High-k dielectrics can provide enhanced capacitance in comparison to an equivalent thickness of silicon dioxide. A metal electrode with suitable work function is desired to avoid charge carrier depletion proximate the electrode interface with the high-k dielectric. The electrodes for p-channel and n-channel transistors may require different metals to provide suitable work functions.
Suitable metals for gate electrodes can be adversely affected by processing used to form source and drain regions. In particular, annealing to repair source and drain implant damage can shift the work function of electrode metals. This has led to various new manufacturing processes, including replacement gate (gate-last) processes. In a replacement gate process, a gate stack is formed with polysilicon in place of the electrode metal. After source and drain regions are formed, the polysilicon is removed to form trenches which are then filled with the desired electrode metals.
Semiconductor devices are subject to thorough testing by manufacturers to ensure they meet performance standards. Devices that fail are discarded or recycled. Devices with high-k dielectrics and metal electrode gates manufactured by preexisting methods fail at a rate that reduces yield to an unacceptable degree. There has been a long felt need for a high yield process for incorporating high-k dielectric, metal electrode gates into semiconductor devices.